1. Field of the Invention
This invention pertains generally to the field of electromagnetic field measurements and more particularly to apparatus for measuring electromagnetic fields using modulated scatterers.
2. Description of the Prior Art
Measurement of the characteristics of the radiated field from the antenna is of practical importance in antenna design, as well as for other purposes including the determination of the characteristics of the medium through which the radiated wave passes. Numerous techniques have been developed to obtain the desired radiated field measurements. See, e.g. W. H. Kummer, et al., "Antenna Measurements--1978," Proceedings of the IEEE, Vol. 66, No. 4, April, 1978. Using transform techniques and computers, it is possible to measure the near-field of an antenna aperture, and predict the far-field with substantial precision. Such measurement techniques eliminate the need for large antenna testing ranges or large anechoic chambers, and allow antennas in the field to be tested quickly after installation.
A common current technique of measuring field distributions involves the use of two mutually orthogonal polarized antennas. The output of each antenna is alternatively switched and each is measured in time sequence using a single radio frequency receiver. Several problems are encountered when using this technique, including the difficulty of completely uncoupling and balancing the two antennas. The radio frequency signals received by the two antennas must be transmitted via waveguides or coaxial cables to a synchronous switch connected to a radio frequency receiver. This additional equipment can affect the field being measured. Generally, the orthogonal antennas can only measure the orthogonal components of the electric field. While this may be sufficient in some applications, if the tilt of the polarization ellipse and the ratio of the major axis to minor axis is desired, then the data must be further processed by a computer.
Electric field distributions have been measured utilizing the back scattered signal from a modulated electric dipole or a spun dipole. However, previous techniques for making such field measurements have not been able to simultaneously determine, in real-time, the magnitude, phase shift, and polarization of the electric or magnetic field at the scatterer.